I have been experiencing a few issues on Sierra.
1. When you hit Play, the black bars that move to show you which bar is playing display erratically for the first few bars. BIAB 2015 with the latest build is running on a brand new MacBook Pro. They don't display in time properly until the third or fourth bar. This means I can't use it to show students without throwing them off.
Try disabling the two options that are found in Realtrack settings and that I've highlighted on the image below.
I'm not sure if this will help because I have BIAB (Windows). We have similar settings in the Windows version and these settings have been known to solve similar problems.
2. When I try to make a some form use a C section, there's no way I can find to actually indicate the C section bars on the chord sheet. Is there a way to do that, please?
It sounds like you are using the (blue) A and (green) B part markers to identify your A and B sections.
While you can certainly use these for that purpose, the role of the A/B part markers is to change sub-styles for a particular number of bars. The B part markers introduce a more energetic playback than the A part markers. This playback style continues until another part marker is encountered. I tend to use A part markers at the beginning of verses and B part markers at the beginning of the choruses.
Part markers also tell BIAB to include a drum break in the bar immediately before the part marker.
It is possible to use Section Text in the notation window and then this will also show up in the Lead Sheet Window. This would allow you to identify as many sections as you need.
Hope this helps,
Noel